Re: [PATCH 09/10] x86-32: use SSE for atomic64_read/set if available

From: Avi Kivity <avi@redhat.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Luca Barbieri <luca@luca-barbieri.com>,
	mingo@elte.hu, a.p.zijlstra@chello.nl, akpm@linux-foundation.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 09/10] x86-32: use SSE for atomic64_read/set if available
Date: Thu, 18 Feb 2010 11:56:35 +0200	[thread overview]
Message-ID: <4B7D0ED3.5020409@redhat.com> (raw)
In-Reply-To: <4B7C8E04.6070605@zytor.com>

On 02/18/2010 02:47 AM, H. Peter Anvin wrote:
>
>>> Unless the performance advantage is provably very compelling, I'm
>>> inclined to say that this is not worth it.
>>>        
>> There is the advantage of not taking the cacheline for writing in atomic64_read.
>> Also locked cmpxchg8b is slow and if we were to restore the TS flag
>> lazily on userspace return, it would significantly improve the
>> function in all cases (with the current code, it depends on how fast
>> the architecture does clts/stts vs lock cmpxchg8b).
>> Of course the big-picture impact depends on the users of the interface.
>>      
> It does, and I would prefer to not take it until there is a user of the
> interface which motivates the performance.  Ingo, do you have a feel for
> how performance-critical this actually is?
>    

One heavy user is set_64() in the pagetable code.  That's already in an 
expensive operation due to the page fault so the impact will be quite 
low, probably.

-- 
error compiling committee.c: too many arguments to function